1. Field of the Invention
The present invention relates to an image pickup apparatus having an image sensor which is provided with color filters, and more particularly to an image pickup apparatus having an improved signal processing for white-balance adjustment.
2. Description of Related Art
FIG. 10 shows in a block diagram the signal processing system of a conventional single-plate type image pickup apparatus. In this case, the image pickup apparatus is assumed to be provided with complementary color filters having such an arrangement as shown in FIG. 2.
Referring to FIG. 10, an analog image signal outputted from an image sensor 401 is supplied through a preprocessing circuit 402 to an A/D converting circuit 403, where the analog signal is converted into a digital signal. The black level of the digital signal is uniformalized by an OB circuit 404 and is then temporarily stored in a memory 405. It is to be noted that the digital signal as mentioned above is composed of signals Wr, Wb, Gr and Gb obtained by performing a generalized addition read-out on the image sensor 401 and adding together respective signals corresponding to color filters Ma, G, Cy and Ye.
The image signal read out from the memory 405 is gain-adjusted by a pixel gain adjusting circuit 406 according to a gain adjustment value obtained from a white balance circuit 420. The gain-adjusted image signal from the pixel gain adjusting circuit 406 is sent to a color-separation and color-interpolation circuit 407, where the image signal is then sent out as signals Wr, Wb, Gr and Gb to a color processing part situated at the next stage, and at the same time, the gain-adjusted image signal from the pixel gain adjusting circuit 406 is also sent to a luminance processing part without altering the arrangment of signals read out from the image sensor 401.
At the color processing part, the output of the color-separation and color-interpolation circuit 407 is sent to a vertical low-pass filter circuit 408. The output of the vertical low-pass filter circuit 408 is sent to a matrix circuit 409 to be converted into color signals R, G and B. The color signals R, G and B thus obtained are sent to a horizontal low-pass filter circuit 410 to have their frequency band restricted. The output of the horizontal low-pass filter circuit 410 is sent to a gamma converting circuit 411 to be subjected to a gamma correcting process. The output of the gamma converting circuit 411 is sent to a pure-color color-difference converting circuit 412 to be converted into a signal Y and color-difference signals R-Y and B-Y. These color-difference signals R-Y and B-Y are sent to the white balance circuit 420 and a C-SUP circuit 413. At the C-SUP circuit 413, the color-difference signals R-Y and B-Y are subjected to a process of killing the color of their saturated areas. The output of the C-SUP circuit 413 is sent to a Y compensating circuit 419, and is also sent to a color compensating circuit 414, which then performs fine color adjustment.
On the other hand, at the luminance processing part, the output of the pixel gain adjusting circuit 405 is processed to correct a difference in luminance brought about by the color filters. This process is carried out through a vertical low-pass filter circuit 415 and a horizontal low-pass filter circuit 416. Then, an edge emphasizing process is performed on the output of the horizontal low-pass filter circuit 416 by a luminance aperture correcting (APC) circuit 417. The output of the APC circuit 417 is sent to a gamma correcting circuit 418 to be subjected to a gamma correction process. A luminance signal thus obtained is sent from the gamma correcting circuit 418 to the Y compensating circuit 419 to have its luminance finely adjusted by using the color-difference signals sent from the C-SUP circuit 413.
The color-difference signals processed by the color processing part and the luminance signal processed by the luminance processing part are temporarily stored in a memory 423. In a case where the image sensor 401 is subjected to the field reading, the field signals are added together into one frame signal by an addition circuit 421. The frame signal is sent to a compressing circuit 422 to be compressed in accordance with the JPEG system or the like and, after that, is recorded in a flash memory or the like.
The white balance circuit 420 has a clipping circuit for receiving the color-difference signals R-Y and B-Y and for limiting inputs having a large amplitude exceeding a predetermined value with respect to the amplitude of the received color-difference signals R-Y and B-Y. The white balance circuit 420 is controlled by a system control CPU to adjust the gain of output values of the image sensor 401 as follows. While one picture is divided into a plurality of blocks, the color-difference signals and the luminance signal which have passed through the clipping circuit are integrated for each of the divided blocks. Then, in a coordinate system formed by the color-difference signals R-Y and B-Y as shown in FIG. 11, a color-temperature varying range of white color is set as a white determining range 501 on the coordinate system. It is determined whether the integrated value obtained at each of the above-stated blocks is within the white determining range 501 on the coordinate system. Then, gains for the R signal and the B signal are decided by using the integrated values which are within the white determining range 501. From the gains, a gain control value for control over the output of the image sensor 401 is obtained. Thus, the gain of output values of the image sensor 401 is adjusted on the basis of the gain control value.
In an image pickup apparatus using a conventional white balance adjusting method in which a white determining range is set on a coordinate system formed by the color difference signals R-Y and B-Y and an integrated value included within the white determining range is used for white balance adjustment, as mentioned above, if the area of an object of red system of low saturation is large at the time of photography under illumination of high color temperature, the illumination of high color temperature would be misjudged as illumination of low color temperature, because the object of red system of low saturation is included in the same area as that of white of low color temperature in the white determining range. Similarly, if the area of an object of blue system of low saturation is large at the time of photography under illumination of low color temperature, the illumination of low color temperature would be misjudged as illumination of high color temperature, because the object of blue system of low saturation is included in the same area as that of white of high color temperature in the white determining range.
Further, in the conventional white balance adjusting method, any color evaluation value that is not included within the white determining range is not used. Otherwise, the color evaluation value that is not included within the white determining range is replaced with the value of an edge part of the white determining range located nearest to that color evaluation value, and the thus-obtained value is integrated as a new color evaluation value. This arrangement necessitates the image pickup apparatus to be provided with an additional computing circuit.
Further, in a case where the conventional image pickup apparatus uses an image sensor which is provided with complementary color filters, the color reproducibility would degrade unless a matrix arithmetic operation is performed in an optimum manner according to color temperature in converting the image signal after white balance adjustment into a pure color signal through the matrix arithmetic operation.
According to the conventional arrangement, the color evaluation value varies with the method of reading from the image sensor. Therefore, it has been necessary to set the white determining range for one method separately from the range set for another method. For example, with the image sensor having the complementary color filters arranged as shown in FIG. 2, the color evaluation value to be used is a value expressed as (R-Y, B-Y) or (((Wrxe2x88x92Gb)xe2x88x92(Wbxe2x88x92Gr))/(Wr+Gb), ((Wrxe2x88x92Gb)+(Wbxe2x88x92Gr))/(Wb+Gr)) in the case of addition read-out, and is a value expressed as ((Yexe2x88x92Cy)/Y1, (Maxe2x88x92G)/Y1) in the case of non-addition read-out. Thus, there exist a plurality of color evaluation values, which necessitate a plurality of white determining ranges to be prepared.
Further, according to the arrangement of the conventional image pickup apparatus described, if the output of the image sensor is inputted to the white balance adjusting part as it is, the number of sample points might become too small in the event of an image having many edges. Besides, in that event, these edges might be misjudged to be white in color. Under such a condition, accurate white detection is impossible.
In view of the above-stated problems, it is a general object of the invention to provide an image pickup apparatus which is capable of accurately detecting color temperature independently of the level of a luminance signal, appositely performing white balance control and reproducing colors in an optimum manner.
To attain the above object, in accordance with an aspect of the invention, there is provided an image pickup apparatus, which comprise an image sensor having a plurality of kinds of color filters, color signal evaluating means for evaluating a color signal outputted from the image sensor, determining means for determining whether the color signal evaluated by the color signal evaluating means is included within a predetermined evaluation range, white balance adjusting means for adjusting white balance on the basis of the color signal determined by the determining means to be included within the predetermined evaluation range, and varying means for varying the evaluation range of the determining means according to a predetermined condition.
In accordance with another aspect of the invention, there is provided an image pickup apparatus, which comprises an image sensor having a plurality of kinds of color filters, first color signal evaluating means for evaluating a color signal outputted from the image sensor, first determining means for determining whether the color signal evaluated by the first color signal evaluating means is included within a predetermined evaluation range, first arithmetic operation means for performing an arithmetic operation to obtain a first control signal on the basis of the color signal determined by the first determining means to be included within the predetermined evaluation range, second color signal evaluating means for dividing a signal for one picture outputted from the image sensor into a plurality of areas, averaging color signals obtained in each of the divided areas and evaluating the averaged color signals, second determining means for determining whether each of the color signals evaluated by the second color signal evaluating means is included within the predetermined evaluation range, second arithmetic operation means for performing an arithmetic operation to obtain a second control signal on the basis of the color signals determined by the second determining means to be included within the predetermined evaluation range, and white balance adjusting means for adjusting white balance on the basis of the first control signal and the second control signal.
In accordance with a further aspect of the invention, there is provided an image pickup apparatus, which comprises an image sensor having a plurality of kinds of color filters, color signal evaluating means for evaluating a color signal outputted from the image sensor, determining means for determining whether the color signal evaluated by the color signal evaluating means is included within one of a plurality of evaluation ranges, weighted-correction means for, if the color signal is determined by the determining means to be included within one of the plurality of evaluation ranges, performing a weighted correction on the color signal according to the evaluation range within which the color signal is included, and white balance adjusting means for adjusting white balance on the basis of the color signal corrected by the weighted-correction means.
The above and other objects and features of the invention will become apparent from the following detailed description of preferred embodiments thereof taken in connection with the accompanying drawings.